TELEDYNE PARADISE DATACOM
APPLICATION NOTE
SAVING SATELLITE BANDWIDTH BY
OPTIMISING SPECTRAL ROLL-OFF
AN_035
1. INTRODUCTION
Teledyne Paradise Datacom has introduced 5%, 10% and 15% spectral roll-off options
(using root-raised-cosine filtering) on all Evolution, Quantum and Q-Series (Q-Flex and QLite) satellite modems with immediate effect.
These new options significantly reduce the required allocated satellite bandwidth thereby
directly reducing the operational expenditure associated with leasing transponder bandwidth.
•
•
•
5% roll-off provides a 20% bandwidth saving compared to 35% roll-off.
5% roll-off provides a 10% bandwidth saving compared to 20% roll-off.
When using the new roll-off factors no increase in Eb/No level is required to
achieve the same BER performance.
The new roll-off factors are available for all satellite services other than DVB-S2 and
SmartLink (which will be supported at a later date). Support for the new roll-off factors is
available for all existing Evolution and Quantum modems as a software-only upgrade (from
software version V2.0.27 onwards).
2. SPECTRAL CHARACTERISTICS
The spectral characteristics as a function of transmit symbol rate are detailed in
Table 1.
RELATIVE
LEVEL
BANDWIDTH NORMALISED TO SYMBOL RATE
35%
Roll-off
25%
Roll-off
20%
Roll-off
15%
Roll-off
10%
Roll-off
5%
Roll-off
-3dB
0.9882
1.0019
0.9946
0.9982
1.0019
0.9982
-25dB
1.3249
1.2378
1.1942
1.1471
1.1071
1.0672
-26dB
1.3294
1.2406
1.1967
1.1490
1.1089
1.0695
-30dB
1.3394
1.2451
1.2015
1.1579
1.1144
1.0745
Table 1 - Measured Modem Tx Output Bandwidth Normalised to Baud Rate
Definitions of terms such as occupied and allocated bandwidth vary. For that reason, Table 1
shows the carrier bandwidth at specific points that allow link budget calculations to use
whichever roll-off factor is relevant. For example, to calculate the bandwidth (in Hz) of a
carrier with a 5% roll-off at the -30dB point, the symbol rate of the carrier should be
multiplied by a factor of 1.0745.
In order to calculate the difference in required bandwidth between two spectral roll-off
factors, use the following equation:
Bandwidth saving (%) = 100 – ((roll-off A / roll-off B) * 100)
2
where A is the lower of the two roll-off factors.
For example, comparing a 5% roll-off to a 35% roll-off at the -30dB point:
Bandwidth saving = 100 – ((1.0745 / 1.3394) * 100)
= 19.7%
3. BANDWIDTH SAVING EXAMPLE
The following real-world example demonstrates the bandwidth saving that was made by
switching a symmetric bi-directional 10Mbps FastLink low-latency LDPC link from a 20%
spectral roll-off to a 5% roll-off.
The link uses two Quantum PD60 modems in the following configuration:
•
•
•
•
•
•
•
•
FEC: FastLink LDPC rate 0.726.
Modulation: 16APSK.
Terrestrial data rate: 10Mbps.
Symbol rate = 3,442,383 symbols = 3.442Msps.
Spectral efficiency: 2.9.
Spectral roll-off factor: 20%.
Carrier bandwidth at -3dB point = 0.9946 * 3.442383 = 3.424MHz.
Carrier bandwidth at -30dB point = 1.2015 * 3.442383 = 4.136MHz.
Now changing the spectral roll-off factor from 20% to 5%:
•
•
•
•
•
Symbol rate = 3,442,383 symbols = 3.442Msps.
Spectral efficiency: 2.9.
Spectral roll-off factor: 5%.
Carrier bandwidth at -3dB point = 0.9982 * 3.442383 = 3.436MHz.
Carrier bandwidth at -30dB point = 1.0745 * 3.442383 = 3.699MHz.
The overall bandwidth saving at the -30dB point from switching from 20% roll-off to 5% rolloff in this case is therefore:
Bandwidth saving = 4.136MHz – 3.699MHz = 0.437MHz per carrier
Total bandwidth saving = 0.437MHz x 2 = 0.874MHz
As a percentage of the original bandwidth of 4.136MHz per carrier, this represents a
saving of 10.5%.
For this C-band service, the bandwidth cost is $3000 per megahertz per month.
Consequently the total cost of the bandwidth for both carriers reduced from $24,816
per month to $22,194 per month, i.e. a saving of $2,622 per month.
3
4. IMPACT ON PEAK-TO-AVERAGE-POWER RATION
The relationship between transmit Peak-to-Average-Power Ratio (PAPR) is detailed in
Table 2.
PAPR (dB) WITH MODULATION SCHEME
ROLL
OFF
BPSK
QPSK
35%
4.1dB
25%
20%
OQPSK
8PSK
8QAM
16QAM 16APSK 32APSK 64QAM
3.9dB
3.8dB
5.3dB
6.0dB
4.8dB
5.6dB
6.5dB
5.1dB
4.9dB
4.7dB
6.0dB
6.6dB
5.7dB
6.3dB
7.1dB
5.7dB
5.4dB
5.2dB
6.4dB
6.9dB
6.1dB
6.7dB
7.3dB
2.2dB
15%
6.4dB
5.9dB
5.7dB
6.7dB
7.2dB
6.6dB
7.1dB
7.6dB
10%
7.2dB
6.4dB
6.1dB
7.2dB
7.6dB
7.0dB
7.5dB
7.8dB
5%
7.9dB
6.8dB
6.6dB
7.6dB
7.9dB
7.4dB
7.9dB
8.1dB
Table 2 - Theoretical Modem Tx PAPR As Function of Modulation Scheme and Roll-Off
The table shows that extra back-off may be required in the uplink HPA when using lower rolloff factors.
The transmit spectral masks for the various roll-off factors are shown in the following
appendix.
4
APPENDIX A – TRANSMIT OUTPUT SPECTRA FOR PARADISE ROLL-OFF FACTORS
Figure 1 - Spectrum For 35% RRC For 1MBaud With -25dB BW Measurement
5
Figure 2 - Spectrum For 25% RRC For 1MBaud With -25dB BW Measurement
6
Figure 3 - Spectrum For 20% RRC For 1MBaud With -25dB BW Measurement
7
Figure 4 - Spectrum For 15% RRC For 1MBaud With -25dB BW Measurement
8
Figure 5 - Spectrum For 10% RRC For 1MBaud With -25dB BW Measurement
9
Figure 6 - Spectrum For 5% RRC For 1MBaud With -25dB BW Measurement
Paradise Datacom Limited
2&3 The Matchyns, London Road, Rivenhall End, Witham,
Essex, CM8 3HA, England
Telephone +44 (0)1376 515 636
Facsimile +44 (0)1376 533 764
email: http://www.paradisedata.com/support.php
www.paradisedata.com
Paradise Datacom LLC
328 Innovation Blvd., State College, PA 16803, U.S.A.
Telephone +1 814 238 3450
Facsimile +1 814 238 3829
email: http://www.paradisedata.com/support.php
www.paradisedata.com
29 August 2012
10